Tuning up your plane insures that it is working at it's maximum potential. A well tuned plane has a flat slippery sole, a well seated frog, a flat bed properly aligned, a mouth with edges square to the body, a properly-positioned, tight-fitting cap iron and sharp blade. Below is a description of tuning up a Stanley Bailey design metal plane.
Some people suggest flattening the sole with the frog removed and then adjusting the way the frog seats in the body so that the plane sole does not deflect. This insures the frog is not stressing the sole. Others flatten the sole with the frog in the body to account for any deflection that the frog causes. I've read recomendations to flatten the sole with the frog mounted and then changing the way the frog seats. This third suggestion seems like the wrong order to do things. The sole will probably not be flat in the end when the frog bolts are tightened. I think that the order of the steps is debatable. Below is the order I will do things in next time I tune up a plane.
Disassemble the plane completely. Clean everything and lubricate the leavers and wheels of the plane.
When you hold a straight edge against the sole you should not be able to slip a 0.003" feeler gauge between the two. Flattening the sole can be done several different ways. You can lap on sand paper glued to a flat surface working from 100 grit paper up to 600 grit. For sand paper you can use a cut sanding belt. In Making & Modifying Woodworking Tools Jim Kingshott suggests using kerosene lubricant on the finer grit papers to help carry away the filings and keep the paper from clogging. As you lap, make sure to use the surface of the sand paper evenly. If the sandpaper wears more in the middle then the sole will end with a hump down the middle of it's length. The flat surface could be a surface plate, granite tile, glass, or a table saw or jointer top. You must be careful not to rock the body side to side or else the sole will round instead of flatten. Another method uses a thin coating of machinist's blue on a surface plate or piece of glass. Press the sole of the plane against the blued surface. Use a machinist's scraper to remove the now blue high spots on the sole. Repeat for the remainder of the day until the entire sole turns blue with a single press down on the reference surface. You do not really have to get the entire surface blue. As long at the front and back of the sole and the areas in front of and behind the mouth are flat and in the same plane then that is o.k. too. However these areas will wear down more quickly than if the whole sole is flat. I believe Japanese woodenplanes have a sole that is purposely shaped like this. Maybe they flatten the soles regularly. You can grind an old file tip to make a scraper. When I did this I was amazed how hard the surface of the sole was initially. Once you break though the patina the scraping goes much faster. Some people want to use power tools and flatten the sole of their plane on a belt sander with success. I imagine you can really mess up fast this way too.
If you will be using your plane for shooting then at least one of the sides (wings) of the plane must be flat and perpendicular to the sole. Check the wings for square. I haven't tried to improve the wings on my planes because I haven't done any shooting. When I see reference to squaring the wings, people usually recommend having a machine shop do this for you with a grinder. I imagine that to do this yourself you could use the same scraping method you can use to scrape the sole. However you need to have a reference right angle which you can coat one face with machinist's blue. With the sole is flat against one reference face, the wing can then be pressed against the blued face to reveal the high spots. Alternately you could attach sand paper to a long piece of wood with a square face and edge. With the face of the block and the sole of the plane on a flat surface you could sand the wing square and flat. It's an idea.
To properly seat the frog, thinly coat the mating surfaces on the plane body with machinist's blue marking fluid. Position the frog and lightly press down. Remove the frog. The blue spots on the frog's mating surfaces need to be filed or scraped with a machinist's scraper. In Making and Modifying Woodworking Tools Jim Kingshott recommends "at least 75% of the surface should blue up for a good fit."
Since the handles bolt to the body, just like the frog, make sure they are in good condition and seat well now so as not to introduce new stresses to the sole.
Bolt the frog and handles to the sole. This should not flex the sole. Check that it remains flat.
If the face of the blade that presses against the bed is not flat then lap it flat now. You can use a bench stone or sandpaper on glass. The face of the blade will serve as a reference for flattening the frog bed. Also it should be flat so it seats well against the frog when in use. This side does not have to have a mirror polish like the other side of the blade does. It just has to be flat enough.
Fattening the frog bed can be done by thinly coating the surface of a flat blade with machinist's blue and lightly pressing it against the bed. The resulting blue areas on the bed are high spots and need to be scraped away with a machinists scraper. The planes of the bed and sole meet in a line. In the end you want this line to be perpendicular to the length of the plane. Check that the front and back edges of the mouth are square to the sole. If the back edge of the mouth becomes sharp then lightly file to remove this sharp edge. This will help avoid chipping the back of the mouth and does not affect the way the blade is supported by the plane because the blade is bevel-down.
Little gaps between the cap iron and the blade will quickly be jammed full of shavings. To insure proper mating you can use a flat abrasive stone or sandpaper on glass. If using a stone, an oil stone might be better than a waterstone as you could quickly wear a groove in a soft waterstone. Hold the cap iron so that it's mating edge is on the stone and near the edge. The other end is hanging off the stone below the level of the stone surface by about 5 degrees. This way, when the blade and cap iron are assembled, the very end of the cap iron will be pressed against the blade first and there will be no gaps.
The cap iron side of the blade must be flat. Lap that flat now. If you are not using back bevels then it should be polished like a mirror for at least 1/8" back from the cutting edge. This is the only part of the blade that will be exposed when the cap iron is attached.
Sharpen the blade. Discussion of sharpening bevel down plane blades.
Assemble the plane. Some slight new stresses may be introduced that flex the sole. In particular, when the lever cap is used to clamp the blade to the frog. Check that the sole is flat. If it is only a little out of flat then reflatten the sole while the plane is assembled as it would be used. Retract the blade slightly so it is just above the sole.
Since the parts of a plane work together tuning the plane could be an iterative process. You could flatten the sole only once at the end and then square the sides but this might mean that the bed and sole do not meet in a line perpendicular to the length of the plane. So you would have to check this and maybe resurface the bed.
Finally, waxing the sole with paraffin wax, paste wax, carnuba wax (the hardest natural wax), Waxilit or Topcote (read about the last two on Lee Valley's site) will make the sole slide smoothly across the wood so that more of your force is directed to the blade. The harder the wax the longer it will wear and the less will be transfered to the wood with each stroak. People do not report problems with wax on the sole affecting finishes but it could. I believe you can wipe something like mineral spirits on the wood before finishing to remove any traces of wax. Also this will reveil any remains of glue or other imperfections that might affect the finish. I have to look into this more. Hoff Stewart posted on rec.woodworking: "Jim Kingshott (who makes furniture for the royal family I believe) uses a small quantity of linseed oil in an oil wick. The wick is in a glass jar in a wood block about 4 by 3 inches. The wick is a rolled up piece of felt. He swipes the plane and takes a few strokes. Works for him, I plan to try it one day." Another person suggested that the oil wick soaked in linseed oil could spontaneously combust so store it in an air tight container.
Test the plane. You can play with the cap iron's distance from the cutting edge and the mouth opening by moving the frog back or forward. For some combination you will make optimal shavings without getting the shavings stuck in the mouth.
After time passes and the plane is used, the soft, cast iron sole will wear and may no longer be flat enough. Also, the internal stresses of cast parts may warp the plane and you will have to re-tune it. Perhaps releasing tension in the cap iron during storage may reduce some potentially warping tension. Steel body planes are longer wearing and avoid the second problem as they are not cast but made of steel plate.
Feedback? Questions?